Issues sending mail to Exchange 2003 mailboxes during Exchange 2003/2010 co-existence

Shortly after upgrading some mailboxes to 2010 it became apparent that when sending emails to Exchange 2003 distribution list. Some of the Exchange 2003 mailbox recipients specified by the distribution list were bouncing with a 5.2.1 NDR message:

"There's a problem with the recipient's mailbox. Please try resending this message. If the problem continues, please contact your helpdesk."

After a some searching I found a great fact finding post on the technet forums: http://social.technet.microsoft.com/Forums/en-US/exchange2010/thread/552513a7-c70b-4c52-b208-173751d393e8/

Further inquiry resulted in these links:

http://social.technet.microsoft.com/Forums/en-US/exchange2010/thread/be90453d-c028-4a02-a5d3-f4eaa1a7556e/

http://technet.microsoft.com/en-us/library/bb851493%28EXCHG.80%29.aspx

This is one of the less common experiences I have had during Co-Existence.